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By Mason Inman Just running an elevator up and down is enough to determine the “structural health” of a building, a new study reveals. The elevator itself might run smoothly up and down, but the heavy counterweights that rumble along the vertical tracks of cable-suspended elevators produce vibrations as they go. This subtle shaking is enough to probe a building for structural weaknesses, according to the new study, led by Robert Nigbor of the University of California, Los Angeles, US. Vibration signals are normally obtained using by gently moving a building with a mechanical “shaker” attached to the structure, or by using vibrations from the environment, such as wind or seismic “micro-tremors”. Both these methods have disadvantages, though. Normally “you have to lug a big shaker up to the top of a building,” Nigbor says, “which is expensive and time-consuming”. Vibrations from the environment meanwhile are hard to control or to replicate accurately. Ironically, the vibration signals produced by moving elevators are typically considered a nuisance for structural testing. “I’ve been doing vibrational measurements in buildings for almost 30 years,” Nigbor says. “You can almost always see the elevator going up and down.” Nigbor and colleagues instead decided to put such elevator vibrations to good use. They say the resulting process that could make vibrational testing much easier in future. The researchers tested the Doris and Louis Factor Building, a 17-storey steel-framed structure on the University of California campus. After a big earthquake hit the area in 1994, the US Geological Survey fitted each floor of the building with motion detectors to measure its response to future tremors. These detectors are sensitive enough to pick up the vibrations from the elevators, their researchers found. By running the elevators up and down repeatedly, on different days, Nigbor and colleagues reproduced identical vibrations, making it easier to spot any structural changes within the building. The researchers found that the vibrations excited the building’s “fundamental modes” of vibration. These are the simplest movements that a building can go through, with the lowest frequency, and are the most revealing when looking for structural damage. One limitation of this method, though, is that the vibrations may change when an elevator goes through maintenance every few years, making it difficult to compare old and new measurements. The approach is “very innovative,” says Sami Masri of the University of Southern California in Los Angeles, US. Although the method requires many sensitive detectors to be installed in the building, at a cost of up to $100,000, Masri says, “relative to the cost of buildings, and to the value of the information you get, it is a very modest investment.” The research was presented at the 6th International Workshop on Structural Health Monitoring, held at Stanford University in California, US,